The conventional process for the synthesis of acrylic acid uses a catalytic reaction of propylene using a mixture comprising oxygen. This reaction is generally carried out in the vapor phase and most often in two stages:
the first stage carries out the substantially quantitative oxidation of the propylene to give a mixture rich in acrolein, in which acrylic acid is a minor component,
the second stage carries out the selective oxidation of the acrolein to give acrylic acid.
The reaction conditions of these two stages, carried out in two reactors in series, are different and require catalysts suited to the reaction. It is not necessary to isolate the acrolein during this two-stage process. The reaction can also be carried out in a single reactor but, in this case, it is necessary to separate and recycle large amounts of acrolein in the oxidation stage.
In a certain number of cases, it may be advantageous to be able to increase the acrylic acid production capacities of existing units.
The production of acrylic acid is highly dependent on the propylene starting material. Propylene, obtained by steam cracking or catalytic cracking of petroleum fractions, has the disadvantage of contributing to increasing the greenhouse effect due to its fossil origin. Furthermore, propylene resources may become limited.
It thus appears particularly advantageous to be able to increase the productive output of acrylic acid while reducing the dependency on a fossil resource.
It has been known for a long time that glycerol can result in the production of acrolein. Glycerol results from the methanolysis of vegetable oils, at the same time as the methyl esters, which are themselves employed in particular as fuels in diesel oil and heating oil. This is a natural product which enjoys a “green” aura, it is available in large amount and can be stored and transported without difficulty. Numerous studies are devoted to giving an economic value to glycerol according to its degree of purity and the dehydration of glycerol to give acrolein is one of the roots envisaged.
The reaction involved in producing acrolein from glycerol is:CH2OH—CHOH—CH2OHCH2═CH—CHO+2H2O
Generally, the hydration reaction is promoted at low temperatures and the dehydration reaction is promoted at high temperatures. In order to obtain acrolein, it is thus necessary to employ a satisfactory temperature and/or a partial vacuum in order to displace the reaction. The reaction can be carried out in the liquid phase or the gas phase. This type of reaction is known to be catalyzed by acids. Various processes for the synthesis of acrolein from glycerol are described in the prior art; mention may in particular be made of the documents FR 695 931, U.S. Pat. No. 2,558,520, WO 99/05085 and U.S. Pat. No. 5,387,720.
The document WO 2005/073160 describes a process for the preparation of acrylic acid from glycerol in two stages, the first stage consisting in subjecting the glycerol to a gas-phase dehydration reaction and the second stage consisting in subjecting the gaseous reaction product thus obtained to a gas-phase oxidation reaction.
It has now been found that the dehydration reaction of glycerol to give acrolein can be carried out in the presence of a gas comprising propylene and more particularly in the presence of the reaction gas resulting from the stage of oxidation of the propylene to give acrolein. It is thus advantageous to introduce glycerol into the conventional process of gas-phase catalytic oxidation of propylene in two stages in order to prepare acrylic acid, which makes it possible to use a renewable starting material while increasing the production of acrylic acid. Such a process then corresponds to the criteria associated with the new concept of “green chemistry” in a more general context of sustainable development.